Potassium (K) channels are important for normal placental function. Activity and expression of a number of K channel isoforms has been reported in syncytial and vascular tissues [1,2]. mRNA expression of KV9.3, a voltage-gated (KV) channel linked to tissue responses to oxygenation [3], has been demonstrated in whole placental homogenate by RT-PCR. However, the channel’s electrophysiological properties and the lack of a human isoform specific antibody have hampered further investigations. We aimed to produce an antibody to human KV9.3, with a view to examining protein expression and localisation in human placental tissues. Term placentas were collected from uncomplicated singleton pregnancies or from women whose pregnancies were affected by fetal growth restriction. An affinity-purified polyclonal antibody was raised in rabbit to residues 131-147 of the human isoform of KV9.3. Channel expression in samples of placental homogenate was confirmed by Western blotting. Protein expression and localisation was investigated using standard immunohistochemical techniques. Scored data were assessed using Mann-Whitney U-test with a threshold of P<0.05 indicative of statistical significance. KV9.3 expression was confirmed by Western blotting. A single discrete band was observed at ~61kDa; signal was ablated by antibody pre-absorption (10X excess of blocking peptide) and incubation with pre-immune serum. Immunohistochemical staining was assessed by three independent researchers using an arbitrary scoring system, blinded to the identity of the tissue. Strong staining was observed in syncytiotrophoblast, particularly localised to the microvillous membrane. Endothelial cells within intermediate and stem villus and chorionic plate blood vessels stained positively for KV9.3, but staining was absent in capillary loops of terminal villi. Vascular smooth muscle cell staining was only apparent in larger diameter vessels. Expression was not significantly altered in tissue from women with fetal growth restriction. Expression of KV9.3 suggests human placental tissues may respond to oxygenation fluctuations via alterations in cell membrane potential. This may have important implications for syncytiotrophoblast cell turnover, nutrient transport and for blood flow in fetoplacental blood vessels.